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Near Earth Asteroids (NEAs)
A Chronology of Milestones Copied from the IAU website http://www.iau.org/public/nea/ Introduction Asteroids, or minor planets, are small and often irregularly shaped celestial bodies. The known majority of them orbit the Sun in the so-called main asteroid belt, between the orbits of the planets Mars and Jupiter. However, due to gravitational perturbations caused by the planets, a continuous migration brings asteroids closer to Sun, thus crossing the orbits of Mars, Earth, Venus and Mercury. An asteroid is coined a Near Earth Asteroid (NEA) when its trajectory brings it within 1.3 AU [Astronomical Unit; for units, see below in section Glossary and Units] from the Sun and hence within 0.3 AU of the Earth's orbit. A NEA is said to be a Potentially Hazardous Asteroid (PHA) when its orbit comes to within 0.05 AU (= 19.5 LD [Lunar Distance] = 7.5 million km) of the Earth's orbit, the so-called Earth Minimum Orbit Intersection Distance (MOID), and has an absolute magnitude H < 22 mag (i.e., its diameter D > 140 m). As of 11 September 2010: *816 NEAs are known with D > 1000 m (H < 18 mag), i.e., 87% of an estimated population of 940 ± 50, including 149 PHAs. *6357 NEAs are known with D < 1000 m, including 988 PHAs with D < 1000 m. The estimated total population of NEAs with D > 140 m (H < 22 mag) is about 25,000. For details, see <[http://neo.jpl.nasa.gov/stats/ neo.jpl.nasa.gov/stats/]>. Several astronomical observatories, at one time or another, have carried out NEO search programs: Lincoln Near-Earth Asteroid Research (LINEAR), USA; Near-Earth Asteroid Tracking (NEAT), USA; Spacewatch, USA; Lowell Observatory Near-Earth Object Search (LONEOS), USA; Catalina Sky Survey (CSS), USA; Japanese Spaceguard Association (JSGA), Japan; Asiago DLR Asteroid Survey (ADAS), Italy/Germany; European NEA Search Observatories (EUNEASO); and La Sagra Sky Survey (LSSS), Spain. Currently the vast majority of NEA discoveries are being carried out by the Catalina Sky Survey near Tucson (AZ, USA), the LINEAR survey near Socorro (NM, USA), and the NEOWISE survey of the Wide-field Infrared Survey Explorer (WISE). A review of NEO surveys is given by: Stephen Larson, 2007, in: A. Milani, G.B. Valsecchi & D. Vokrouhlický (eds.), Proceedings IAU Symposium No. 236, Near Earth Objects, our Celestial Neighbors: Opportunity and Risk, Prague (Czech Republic) 14-18 August 2006 (Cambridge: CUP), p. 323, "Current NEO surveys." The following chronology lists (a) data of known NEAs with past nominal Earth close approach distances d < 1.0 LD; (b) data of know NEAs with future nominal Earth close approach distances d < 10.0 LD and minimum close approach distances d < 1.0 LD; (c) milestones of NEO/NEA research. Information of category (a) is quoted from the NASA JPL NEO Program Close Approach Tables for the period 1900 – 2200 A.D. as of 11 September 2010, available at <[http://neo.jpl.nasa.gov/ca/ neo.jpl.nasa.gov/ca/]>. By listing in chronological order this broad selection of milestones on asteroid research and in particular NEA research, an impression is offered of what has been done and what is being done in those fields. Glossary and Units ---- Chronology Acknowledgements Valuable comments and additions to earlier versions of this document have been gratefully received from Johannes Anderson (Denmark), Daniel C. Boice (USA), Roger M. Bonnet (France), Richard Crowther (UK), Gerhard J. Hahn (Germany), Alan W. Harris (Germany), Alan W. Harris (USA), Ingrid van Houten-Groeneveld (Netherlands), Claus Madsen (Germany), Brian G. Marsden (USA), Andrea Milani (Italy), Thomas H. Morgan (USA), Hans Rickman (Sweden), Russell L. Schweickart (USA), Timothy B. Spahr (USA), Frans G. von der Dunk (USA), I.P. Williams (UK), Ip Wing-Huen (China Taipei), and, in particular, Donald K. Yeomans (USA). Web page management by Raquel Yumi Shida (ESO) is greatly appreciated. External Links=